Helminthofauna of the Sea Trout Salmo trutta Linnaeus, 1758 from the Southern Baltic Sea Region, Including Molecular Characteristics of the Dominant Tapeworm Species Eubothrium crassum (Bloch, 1779)

The sea trout Salmo trutta has considerable ecological and economic importance in the Baltic Sea basin. In recent years, the landings of this species from the Main Basin have decreased significantly. Consequently, it is necessary to consider whether this decline has been associated with any changes in the diversity and domination of its parasitofauna. We examined 101 sea trout specimens originating from the southern Baltic Sea basin and its tributary (the Gulf of Gdańsk and the River Reda) collected between 2003 and 2020. The fish were infected with five parasite species: one tapeworm (Eubothrium crassum), one digenean (Brachyphallus crenatus), and three nematodes (Anisakis simplex, Hysterothylacium aduncum, and Raphidascaris acus). Overall parasite prevalence was high (parasites were found in 94.7% of sea trout), with a mean intensity of 37.4 and a range of 1–125 parasites per fish. Eubothrium crassum, a species widely distributed in Europe and North America, clearly dominated the parasite community: a total of 3345 specimens were recorded in 92.6% of fish, with a mean intensity of 38.0 and an intensity range of 1–125 individuals. Correct morpho-anatomical identification was confirmed by molecular methods. The tapeworms were located primarily (96%) in the pyloric caeca. Other parasite species occurred only sporadically. Infection levels increased with both the length and mass of the fish; however, despite the high parasite infection, no deterioration in the host was indicated by Fulton’s condition factor. Comparison of fish originating from the different time periods revealed no changes in their parasitological characteristics, suggesting that parasite–host interactions are unlikely to have contributed to the decline in fish catches.